The present invention relates to process control systems. More specifically, the invention relates to transmission of process variables from field transmitters to control room circuitry in process control systems.
Generally, process variable field transmitters are used in process control systems to measure process variables such as flow, temperature or pressure and transmit these variables to a control room. This information is used at the control room to monitor and control operation of the process.
Traditionally, transmission of process variables has been through the use of two-wire current loops. A 4 mA signal in a typical 4-20 mA current loop represented a minimum reading and a 20 mA signal represented a maximum reading of the process variable. Power for the current loop and field transmitter was supplied from the control room in order to maintain an intrinsically safe and explosion-proof environment in the field. The field transmitter controlled the current flowing through the loop based upon the measured process variable. This, and other types of current loops, have become industry standards.
With the advent of microprocessors, field transmitters have become increasingly automated. One offshoot of this technology is the use of digital signal transmissions from field transmitters. In one design, a digital signal is superimposed on the 4-20 mA process variable current signal. This digital signal can represent a primary process variable typically also represented by the analog loop circuits or secondary process variables. In another configuration, multiple field transmitters are connected to the same process control loop and each transmitter transmits digital process variables over the loop to the control room. Traditional control room circuitry adapted for the analog communication techniques have not been able to fully utilize the transmitters capable of transmitting digital signals.